GB2256278A

GB2256278A - Testing electrical characteristics of materials

Info

Publication number: GB2256278A
Application number: GB9107151A
Authority: GB
Inventors: Keith Hemingway
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-04-05
Filing date: 1991-04-05
Publication date: 1992-12-02
Also published as: GB9107151D0

Abstract

In an apparatus for testing electrical characteristics of a material, e.g. the conductivity (g) of milk with a view to sensing mastitis, an alternating signal (X1) depending (X2) on the value of a reference signal Vref is applied (X3) to the material, and the signal I passing through the material is rectified (X4) and applied (S1) to an A-D converter (X6) which produces an output representing the relative value I : Vref. Since I = gVref the output thus represents conductivity (g) alone, being displayed at X5. The output from the rectifier (X4) is also compared (X7) with a pre-settable (S2, X10, X9) threshold to activate an alarm (X8), the threshold being varied from one cow to another as desired. <IMAGE>

Description

APPARATUS FOR TESTING AN ELECTRICAL CHARACTERISTIC OF A MATERIAL.

According to one aspect of the invention, there is provided apparatus for testing an electrical characteristic of a material, the apparatus comprising: reference signal supply means for supplying a reference signal; alternating signal supply means connected to the reference signal supply means, for supplying an alternating signal dependent upon said reference signal; electrically conductive means connected to the alternating signal supply means for passing said alternating signal through said material to derive a characteristic indicative signal having a value dependent upon said reference signal and upon said electrical characteristic of said material; and output response producing means connected to the reference signal supply means and to said electrically conductive means and operable for producing an output response in dependence upon the value of the characteristic indicative signal relative to said reference signal.

According to a second aspect, there is provided apparatus for testing an electrical characteristic of material, the apparatus comprising supply means for supplying an alternating electrical signal; electrically conductive means for passing the signal through said material to derive a characteristic indicative signal and output response means for supplying an output response dependent upon the value of the characteristic indicative signal characterised in that: the apparatus comprises means for supplying a reference signal (Vref); and the alternating signal supply means (X1, X2) and the output response means (X4 to X10) are each connected to receive the reference signal for the alternating signal to be dependent upon the reference signal and for the output response to be dependent upon said value (I) relative to the reference signal.

It is well known to those involved in biological chemistry that the electrical conductivity of a substance is affected by changes in its chemical content.

In particular, the electrical conductivity of milk for example cow's milk, increases when the cow is beginning to be infected with mastitis and certain other diseases.

The mastitis condition is serious if fully developed, requiring antibiotic treatment and the consequent ban on the sale of milk from the animal until the cure is complete.

Moreover the condition by then is likely to have infected other cows in the herd.

A measuring apparatus which can detect early signs of mastitis (i.e. sub-clinically) is therefore useful in enabling prophylatic treatment without antibiotics to be carried out, resulting in the reduction in veterinary expense and avoiding the loss of milk sales.

The apparatus to be described can be permanently connected on-line in the milk-tube to give a continuous indication of the milk conductivity during milking, the milk passing through a measurement chamber of known geometry containing electrodes.

Previous measurements of this kind have used a broadly similar method but with inadequate accuracy for critical detection, which needs to be of the order of 2%. The method then used was to apply an alternative waveform through a fixed resistance R to the chambers electrodes, as illustrated in Fig. 1. The voltage appearing across R, labelled Vac, was then rectified to become Vdc and compared in the comparator with a fixed reference voltage Vref, set during manufacture.

If Vdc exceeded Vref, audible and visible alarms were activated. In the use described above, milk from this now suspect cow would then be sent for clinical analysis to check for signs of mastitis or other disease.

There were two important deficiencies with the way this method was engineered.

1. The value of Vref was fixed during manufacture at a compromise value intended to suit all cows: in fact each cows milk is likely to be unique, so that some cows' outputs would always cause an alarm even when healthy, whilst others would never result in alarms even when diseased.

2. With practically achievable accuracy, using the method described, the measurement was inadequate to give correct results over a typical range of ambient temperatures encountered in milking parlours. This was mainly due to variations in the amplitudes of the input ac waveform, to the value of Vref and errors in the rectification process (conversion from Vac to Vdc in Fig. 1).

In the apparatus of Fig. 2 the conductivity may be measured more accurately and is continuously displayed as a three-digit number. Also a visible and audible alarm is provided, which operates when reading exceeds a number settable by the user, using the same display. In this way an individual cow's readings can be observed over a period and a suitable alarm threshold set to warn of a rise in conductivity from the usual value for that cow.

In Fig. 2, the amplitude of oscillator X1 is clipped so its excursion limits are OV and +Vref, using an accurate clipper X2. This clipped waveform is applied directly to one of the electrodes in the measurement chamber X3. The other electrode is directly connected to the input of rectifier circuit X4. The rectified output Vdc is directly proportional to R1 and to Iin, and therefore to conductivity G.

Inaccuracies in Vdc, as a measure of g, are due to variations in R1 and Vref. It is easy to obtain highly accurate resistors, but only expensive reference devices have adequate stability against temperature variations.

In the conversion process from Vdc to the three digit display X5, however, a reference voltage is also required to operate the A-D converter X4. By using the same reference source for the two parts of the circuit, namely for the oscillator clipper and for the A-D reference, changes in the value of Vref are automatically cancelled and the displayed reading is therefore independent of Vref, provided Vref lies between certain practical limits.

When Vdc exceeds Vtrip, thereby tripping the comparator X7, the alarm X8 is activated. The trip level Vtrip is derived from the D-A converter X9 and is designed to be adjustable, by connecting the oscillator X1 to counter X10, using switch S2. The value of Vref, with S2 closed, cycles over a range of values designed to correspond with practical requirements. When the desired level is reached, the release of S2 retains the figures.

During this setting process, and at any other time, the trip level will be displayed on X5 if S1 is operated.

For accurate tripping operation, the trip level also moves in the same way as Vref, and this is ensured by again using Vref as the reference voltage for converter X9.

In thee figure 2 apparatus, the use of the same Vref in three parts of the circuit is such that variations in Vref cancel and do not affect either the reading or the alarm trip level. Also the clipping circuit X2 ensures that the oscillator output amplitude, connected to chamber X3, is virtually equal to Vref.

Claims

1. Apparatus for testing an electrical characteristic of a material, the apparatus comprising: reference signal supply means for supplying a reference signal; alternating signal supply means connected to the reference signal supply means, for supplying an alternating signal dependent upon said reference signal; electrically conductive means connected to the alternating signal supply means for passing said alternating signal through said material to derive a characteristic indicative signal having a value dependent upon said reference signal and upon said electrical characteristic of said material; and output response producing means connected to the reference signal supply means and to said electrically conductive means and operable for producing an output response in dependence upon the value of the characteristic indicative signal relative to said reference signal.

2. Apparatus for testing an electrical characteristic of material, the apparatus comprising supply means for supplying an alternating electrical signal electrically conductive means for passing the signal through said material to derive a characteristic indicative signal and output response means for supplying an output response dependent upon the value of the characteristic indicative signal characterised in that: the apparatus comprises means for supplying a reference signal (Vref); and the alternating signal supply means (X1, X2) and the output response means (X4 to X10) are each connected to receive the reference signal for the alternating signal to be dependent upon the reference signal and for the output response to be dependent upon said value (I) relative to the reference signal.

3. Apparatus according to claim 1 or 2, wherein the output response producing means comprises display means for displaying the value of the characteristic indicative signal relative to said reference signal.

4. Apparatus according to claim 3, wherein the output response producing means comprises a rectifier for rectifying the characteristic indicative signal, analog to digital converter means connected to the rectifier and the reference signal supply means for supplying a digital signal representative of the value of the rectified characteristic indicative signal relative to the reference signal1 and digital signal display means for displaying the value of the digital signal.

5. Apparatus according to claims 1,2, 3 or 4, wherein the output response producing means comprises threshold level supply means for making available a threshold signal level dependent upon the reference signal, comparator means for comparing the characteristic indicative signal with the threshold signal level, and alarm means connected to be controlled by the comparator means.

6. Apparatus according to claim 4, wherein said threshold level is operator adjustable.

7. Apparatus according to claim 6 wherein the threshold level supply means comprises: counter means connected to said alternating signal supply means by way of a threshold level setting switch; and digital to analog converter means connected to the counter means and the reference signal supply means for supplying an analog signal having a value representative of the value of the counter content relative to the reference signal.

8. Apparatus according to claim 6 or 7, when appended to claim 2 or 3, including switch means connected between the said display means and the threshold value supply means for rendering the display means operable to display the threshold value.

9. Apparatus according to any preceding claim wherein the alternating signal supply means comprises a limiter for receiving an oscillator signal and hand limiting that signal to the magnitude of the reference signal.

10. Apparatus according to any preceding claim, adapted to test the electrical conductivity of liquid.

11. Testing apparatus substantially as hereinbefore described with reference to figure 2 of the accompanying drawing.